You are starting with the wrong info. Flash shortens the exposure time, meaning you can use very fast shutter speed. Well, if you google it, you will find many tips on flash photography, find the basic one first.
For hands-on, in the meantime, set your shutter speed to the fastest your digital camera can support with flash.
Without changing shutter speed in M mode:
1. Just change aperture 1 stop up or down, shoot with flash. Study your picture, learn ....
2. Repeat No. 1, keep learning, try to understand
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